Selenium toxicity is thought to involve the incorporation of selenoamino acids into proteins. The protein synthesizing machinery of selenium-tolerant organisms, such as the selenium accumulating Astragali, are believed unable to recognize selenocysteine and selenomethionine, resulting in the exclusion of these selenoamino acids from proteins. Cell-free protein synthesizing systems will be prepared with polysomes and post-polysomal fractions from selenium accumulator species to determine whether selenoamino acids are excluded from proteins; similar cell-free systems will be prepared from non-accumulator species. Heterologous systems will be used to study the site of exclusion of the selenoamino acids along the protein synthesizing sequence. The possibility of a specific pathway for selenium assimilation existing in those bacteria which possess selenoenzymes, will be investigated by studying the transport and metabolism of selenate, selenite, selenocysteine and selenomethionine by Escherichia coli.